1. Field of the Invention
The present invention is directed to a method and apparatus for adaptively characterizing and calibrating color document scanners, color display units and color printers and, more particularly, the present invention is directed to providing-unique calibration transform for each device using a low cost achromatic sensor and where the calibration transform minimizes device population variance by providing a method, executable by the user of a color management system, producing constrained channel independent linearization, aim or target curve scaling to achieve a desired color balance and aim curve scaling for channel saturation correction.
2. Description of the Related Art
In desk top (low cost) color image processing or management systems which include peripheral devices, such as document scanners, color cathode ray tube monitors or displays and color printers, consistent color appearance matching between peripheral devices is necessary. Precise color rendering throughout such systems relies on the color stability, consistency and accuracy of the peripheral devices.
Calibration is an integral part of a low cost color management system. Numerous manufacturers market devices and/or software packages which addresses the peripheral calibration requirement with various levels of calibration accuracy. A majority of these solutions are limited to monochrome calibration for cost reasons, and thus are not as accurate as those products incorporating full colorimetric measurement and correction. It is generally though erroneously felt that monochrome calibration is sufficient for the desk top environment.
For scanner calibration, Savitar, markets two solutions aimed at the high and low end markets. The high-end solution, sold under the name SpectraPlate/35, utilizes a numerical analysis package in conjunction with a target containing a linear variable spectral filter and a neutral density gray ramp. The neutral density ramp is used to measure and correct tone scale deficiencies. This ramp exhibits constant transmission characteristics across the visible spectrum and varies linearly by two orders of magnitude in transmission from D-min to D-max. The linear variable filter is utilized to balance the scanner's red, green and blue (RGB) channels to neutral gray across the entire dynamic range of the device. It functions as an interference wedge that spectrally separates the scanner's illumination system into a continuous distribution (in a manner similar to a prism) which is then imaged onto a charge coupled device (CCD) imager. Fiducial marks on the target permits correlation of pixel data to wavelength. This enables the scanner to obtain its spectral response without external instrumentation. Savitar's low end product, ScanMatch, utilizes a 24 patch colorimetric target as a reference, thus enabling accurate colorimetric calibration with in target tolerance. Under this approach, the target is scanned by the device to be calibrated using either the ScanMatch application or some other program containing the appropriate device drivers. The resulting digital data file is processed by the ScanMatch application where data extraction and transform generation occurs. Transform table construction is simply performed by building a model linking scanned data to previously defined reference data that was derived from the target.
Vendors such as RasterOps, Radius, SuperMac and Barco market closed-looped monitor calibration packages which include various combinations of instrumentation (typically a colorimeter), signal correction electronics and control software for automated data acquisition and numerical analysis. Those systems contain or are capable of supporting colorimeters to provide color balance calibration by modifying look-up tables or a color correction matrix. Colorimetric calibration yields optimal results, but is excessively cost prohibitive, therefore, is usually instituted on high end systems. Those packages that do not support this calibration assume a nominal phosphor characteristic in their correction matrix.
The Radius and SuperMac Technology's SuperMatch products provide a low end solution limited to monochrome calibration. Both packages, which are essentially similar, contain a control and numerical analysis application and a measurement device for gun response measurements. These two solutions generate numerous test patches along the primary scales which are subsequently measured to ascertain the independent gun response. A look-up table for each gun is calculated which appropriately distorts incoming data to overcome channel irregularities to form some predetermined, idealized response. This process is generally referred to as "gamma" correction since it negates the monitor's characteristic gamma response, but the proper idealized response could help overcome systems problems such as flair, surround effect, etc. Both products also permit user selection of monitor white point which is accomplished by selecting the desired correlated color temperature and monitor type. Prestored phosphor chromaticity data for each monitor type is utilized to adjust the gamma correction curves to achieve the desired color balance. Obviously, this colorimetric calibration method is inferior when compared to others since device measurements are not performed on each device, instead they are made at the factory on a limited population.
If more exacting colorimetric calibration is required, each device must be measured independently using a colorimeter or spectralradiometer. The RasterOps CorrectColor Calibrator product serves this market by providing an integrated colorimeter with a supporting control/numerical analysis software package. This system supports user defined gamma correction and color temperature adjustments as outlined above, but uses measured phosphor chromaticity values rather than a factory standard default. The overall colorimetric calibration accuracy is then limited by measurement precision.
Printer calibration also utilizes a measurement device (usually a densitometer, colorimeter with a source or a spectrophotometer) and a software package to perform data generation and numerical analysis. The typical calibration procedure includes printing a standardized target containing tone and/or gray scales followed by patch measurement and data insertion into the software (usually automated). Some systems utilize visual matching methods in place of instrumentation which undoubtedly causes higher levels of measurement error as compared with other forms of instrumentation. Numerical analysis is then performed to generate channel independent look-up tables which correct for non-idealized printing characteristics. The exact target characteristic response is a function of the package and is defined linearly with respect to dot gain or in terms of a D-log E curve. This procedure is followed on both 3 and 4 color printers. The 4 color or "black" printer presents a special problem since the manufacturer has optimized the printing process by adding a black component in place of varying amounts of CMY (Cyan, Magenta, Yellow). These UCR (Under Color Removal) and GCR (Gray Component Replacement) algorithms are generally different for each device type and are considered proprietary knowledge of the print engine manufacturer. This information is not readily disseminated which forces the calibration system to compromise for a particular device to support all such devices.
Attempts to market low-end printer calibration tools have been limited at this point because of instrumentation cost and the recent emergence of consumer grade color printers. A need for higher accuracy rendering is just starting to emerge, thus product development efforts have not matured. Eastman Kodak Co. currently sells a calibration package to linearize a QMS (Color Script 100) printer from densitometer measurements of the tone and gray scales.